Fluorescent lamps are the most popular light sources in our life. Improving the efficiency of the fluorescent lamps will significantly save energy. In recent development, how to improve the efficiency and save the power for the ballast of the fluorescent lamp is the major concern, and further preheating filament before lamp ignition will help filament generate free electronics more easily and this can not only reduce ignition voltage between two ends of cathodes but also improve the lifetime of the lamps. Therefore many electronic ballasts or integrated circuit controllers are added preheating filament function for better lifetime of lamps, but this still brings out another problem “repreheat”. Repreheat means preheat the filament more than once during a short time of power tripped for ballast, because during this time the filament is still at high temperature around 1000° K and the short time of power tripped will let ballast reset its function and repreheat the filament again, this will gives two times of energy for filament and occur over preheat. Over preheat will reduce the lifetime of lamps and it should be prevent such situation happen.
Filament with perfect preheating will reduce the ignition voltage at two ends of cathodes and ignition time. Meanwhile if ignition time takes too long to ignite the lamp and high lamp voltage occurred during this time, it will also influence the lifetime of lamps, so it should also avoid.
With preheating filament function, most of conventional electronic ballasts are paralleled one capacitor as a starting capacitor to the lamp to achieve preheat filament before lamp ignition. But glow current is occurred during lamp preheating because the voltage drop between the capacitor and this will reduce the lifetime of lamps.
FIG. 1 shows first conventional series resonant circuit of electronic ballast with preheating filament function using an integrated circuit controller 2. A half-bridge inverter 3 consists of two switches 31 and 32 that controlled by signals S2 and S3 from the integrated circuit controller 2. These two switches 31 and 32 are complementarily switched on and off with about 50% duty cycle at the desires switching frequency controlled by a resistor 12 and a capacitor 14. The resonant circuit is composed of an inductor 40, a capacitor 41 and a fluorescent lamp 50. The fluorescent lamp 50 is in paralleled with a capacitor 51. The capacitor 51 is operated as a starting capacitor. A preheat circuit 1 consists of a logic circuit 11, the resistor 12, the capacitor 14, and a switch 15 connected in series with a resistor 13 paralleled to the resistor 12. The preheat function is done by controlling the switch 15 to parallel the resistor 13 with the resistor 12 for higher frequency switching when a switching signal S1 appears. Preheat time is controlled by the logic circuit 11 before lamp ignition. High starting frequency is to avoid stress on the lamp filament at startup and reduce the ignition voltage on lamps.
In this topology when the DC bus voltage tripped or user switching the power switch in a short time during the lamp operation will let the integrated circuit controller 2 and the preheat circuit 1 to fail its function and repreheat the filament again. Therefore gives filament two times of energy for preheating and this should be avoid. Meanwhile ignition time does not control in this case. If ignition time takes too long to ignite the lamp 50 means will have a higher voltage drop between two ends of cathodes at the same time, this will damage the filament and reduce the lifetime for lamps. It should be also prevent.
Another conventional electronic ballast with preheat function is showed in FIG. 2. A capacitor 61 is coupled to the integrated circuit controller 2 for preheat time control, a capacitor 62 is coupled to the integrated circuit controller 2 for ignition time control. In this case it has to use two extra capacitors for preheating and ignition time control.
The objective of the present invention is to provide ballast with repreheating and ignition time control and necessary signals by using one capacitor. Another objective of the present invention is to develop a low cost circuit for high efficiency performance.